Conventional integrated circuits transmit signals by conducting electrons through semiconducting materials. In a pure silicon crystal, for example, the silicon atoms are arranged in a highly-ordered lattice structure. Electrons moving through this lattice experience periodic potentials as they interact with the silicon nuclei, resulting in the formation of a band gap between a conduction band and a valence band. Since no electrons will have an energy in the band gap of pure silicon crystals, the silicon is doped with suitable impurities to disrupt the periodicity of the crystal lattice. The doping of the silicon crystal can give rise to the absence of a silicon atom or the presence of an impurity at a silicon site in the lattice, or an impurity atom located at a non-silicon site within the lattice structure. In this way, electrons are permitted to have an energy within the band gap, thereby minimizing the size of the energy gap that electrons must surmount in order to make the transition from the valence band to the conduction band, and improving the overall conductivity of the silicon crystal.
The field of photonics aims to develop circuitry that harnesses light to transmit signals and supersede in function the utilities currently carried out by electrons. For example, a photonic crystal can include a block of dielectric transparent material having air holes interspersed throughout said material. The refractive index of the dielectric material is large relative to the refractive index of the air. Photons passing through the block of dielectric material will periodically encounter regions of high refractive index and low refractive index in a manner analogous to electrons periodically encountering allowable energy ranges and the band gap as they pass through a silicon crystal. A large contrast between the refractive index of the dielectric material and the air holes causes most of the light passing through the photonic crystal to be confined within the dielectric material, essentially forming an allowable energy region separated by forbidden energy regions, which are commonly referred to as the photonic band gap. Just as it is possible to create allowable energy levels in the band gap of silicon crystals, it is also possible to create allowable energy levels within the photonic band gap by altering the size of some of the air holes interspersed through the dielectric material.
Though photonics is theoretically sound, there are practical obstacles preventing its widespread application. The construction of photonic crystals requires regular fine features on the length scale of hundreds of nanometers formed with materials having a high refractive index relative to that of air. Photonic crystals made with semiconducting materials and processing methods have been considered for development of photonic crystals. Although semiconducting materials and their processing methods can be used to manufacture regular structures having high refractive indices, the materials have only a limited set of physical properties, and the conventional processing methods for such materials are capital intensive.
Polymeric materials have been proposed for use in the construction of photonic crystals to both expand the physical properties of such devices and reduce the capital requirements in their production. Unlike photonic crystals manufactured from semiconducting materials, however, photonic crystals made from polymeric materials possess a relatively low refractive index contrast, which is usually the ratio of the refractive index of the polymeric material to the refractive index of the air. Typically, organic materials have a refractive index of 1.3 to 1.7, which is lower than desired for photonic crystal fabrication.
It has been proposed that certain conjugated organic polymers, such as polythiophene (“PT”), could be synthesized with a high refractive index of about 1.7, but experimental tests disclosed in A. Hamnett and A. R. Hillman in the J. Electrochem. Soc. 1988, 135, 2517, revealed that the refractive index of polythiophene was actually 1.4. It was hypothesized that the lower-than-expected refractive index was caused by what is referred to as the “Polythiophene Paradox.” This paradox arises because polythiophene (PT) is electrosynthesized and the potential required to polymerize the starting material is greater than the potential required to degrade the resultant polymer. This results in a highly degraded polymer with very low levels of conjugation.
Thus, there is a need in the art for organic polymers having elevated indices of refraction and for methods of making such polymers that permit one to obtain elevated indices of refraction.